1. Field of the Invention
The present invention relates to a multiband antenna having an antenna element which splits into, for example, two or more branches and a radio communication terminal using the multiband antenna.
2. Description of the Related Art
As an example of a multiband antenna, a dual band antenna of what is called a two-branch (two-branch antenna element) configuration, for example, having a single feeding point and configured to be operable in two bands is well known.
FIG. 11 shows a schematic structural example of an existing dual band antenna of the two-branch and single-feeding-point configuration.
The dual band antenna shown in FIG. 11 has first and second antenna elements 101 and 102 and one feeding point 104. One end of each of the first and second antenna elements 101 and 102 is formed into an open end and the other end of each antenna element is connected to the single feeding point 104. The antenna length of the first antenna element 101 is different from that of the second antenna element 102. For example, in the case that the antenna length of the second antenna element 102 is shorter than that of the first antenna element 101, the first antenna element 101 operates as an antenna on the side of a low frequency band and the second antenna element 102 operates as an antenna on the side of a high frequency band.
The dual band antenna of the configuration shown in FIG. 11 is basically constructed such that respective branches resonate at respective frequencies in the above mentioned two bands. However, it is constructed such that the feeding point is commonly used by the antenna elements, so that one antenna element operates as an open stub of the other antenna element. In addition, in the case of the antenna of the configuration shown in FIG. 11, the antenna elements on the lower frequency band side and the higher frequency band side are capacitive-coupled together in their respective operation bands, so that it is not the case that the antenna elements operate completely independently.
In addition, as an existing multiband antenna configured to cope with a plurality of bands, for example by using a single antenna element, there is also well known an antenna in which an LC parallel resonance circuit is provided on an intermediate part of an antenna element. In the case that the LC parallel resonance circuit is provided on the intermediate part of the antenna element, its impedance is open-circuited at a resonance frequency of parallel resonance, so that current hardly flows to the open end side of the antenna element beyond the LC parallel resonance circuit. That is, the antenna of this configuration is an antenna of the type utilizing such a phenomenon that the antenna element seems to be electrically short at a frequency on the high band side and configured to widen the band, in particular, on the high frequency side. Incidentally, the LC parallel resonance circuit used in this structural example operates similarly to what is called a trap (wave trap) circuit. In many cases, the above mentioned LC parallel resonance circuit is designed to be open-circuited on the high band side and is installed at a position apart from the feeding point by an amount corresponding to λ/4 (λ is a wavelength) the high band side frequency.
In addition, for example, in a mobile phone terminal of what is called a GSM (Global System for Mobile Communications) system, an antenna whose wide-band characteristics may be demanded in a frequency band on the high band side is adopted so as to cope with a triple band of 900 MHz/1800 MHz/1900 MHz. Further, recently, there has been also adopted an antenna configured to cope with the UMTS (Universal Mobile Telecommunications System) band 1 (Tx: 1920-1980 MHz, Rx: 2110-2170 MHz) of the third generation mobile phone standard (3GPP). Specifically, an antenna configured such that a parasitic element having the quarter-wavelength λ/4 relative to a high band side frequency is disposed in the vicinity of a two-branch antenna element so as to attain a plurality of tunings has been widely adopted.
In addition, for example, Japanese Laid-Open Patent Publication No. 2004-266311 discloses an antenna including a linear main radiating conductor section having one end constituting a feeding end and the other end constituting an open end and a linear short-circuiting conductor section which branches from an intermediate part of the main radiating conductor section in a T-shaped form and is connected to a grounding conductor. In the above mentioned antenna, the distribution route of antenna current is constituted by a first route running from one end to the other end of the main radiating conductor section, a second current route running from one end of the main radiating conductor section to the grounding conductor via a T-shaped branch and a third route turning back from the other end of the main radiating conductor section to the grounding conductor. Owing to this arrangement, in the above mentioned antenna, at least two resonance frequency bands are obtained at frequencies other than higher harmonics.